Densitometric measuring devices and systems are often equipped with polarization filters. During measurements with polarization filters, a small color density decrease results during the drying of the printing ink and an expanded range in which the color density is linearly correlated with the layer thickness of the printing ink. When the measurement result is used for the machine control during printing, the printer has a large interest to obtain measurement values as long as possible before the color is dry in order to reduce waste. A polarization device is therefor necessary.
Wet printing inks have a shiny surface, while during drying the printing ink adapts to the surface structure of the paper and, depending on the paper quality, becomes more or less matte. This results in different measurement results before and after drying. The purpose of the polarization filter is to suppress the surface reflection of the wet print. Two polarization filters are used therefor. A first polarization filter is positioned in the light path of the light source and therefore lets only one oscillation plane pass through. The oriented light rays are partially reflected at the color surface without a change in the oscillation plane. A second polarization filter is located in the light path of the sensor and is rotated 90° relative to the first filter so that the polarized light rays reflected on a smooth surface cannot pass this second polarization filter or are only partially let through by this second polarization filter. The light portion originating from the surface shine is thereby strongly or ideally totally surppressed and the sensor essentially measures only the unpolarized light portion.
In colorimetric measurement systems, the measured value relates to the human eye. Since measurements with polarization filters do not correlate with the impression of the observer, the standards require color measurements without polarization filters.
Many modem measurement devices offer both possibilities of measurement, namely density measurements and colorimetric measurements, for example according to CIEL*a*b* or CIEL*u*v*. Mechanical solutions are provided in those measurement devices with which the polarization filter effect can be switched on or off.
In a known measurement device, one of the two polarization filters is physically rotatedly positioned so that its polarization plane can be adjusted to be selectively perpendicular or parallel to that of the other polarization filter. At parallel orientation, the light portion of the surface shine can reach the sensor and the measured result corresponds to that of a measurement device without polarization filter. In contrast, at perpendicular orientation of the two polarization planes, the shine portion is filtered out as described. Solutions with physically rotatable polarization filters are however especially disadvantageous for several reasons when the measurement device or system is integrated into a printing machine. However, significant effort is required even with measurement tables or portable apparatus, to ensure that switching of the polarization filters is not forgotten.